Inverters have become installed in many types of electrical equipment, as efficient variable-speed control units. However, inverters are switched at a frequency of several kHz to several ten kHz, to cause a surge voltage at every pulse thereof. Such an inverter surge occurs reflection at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire, in the propagation system, and consequently, to apply a voltage twice as high as the inverter output voltage at the maximum. In particular, an output pulse occurred due to a high-speed switching device, such as an insulated gate bipolar transistor (IGBT), is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipments, such as inverter-related equipments, for example, high-speed switching devices, inverter motors, and transformers, use is made of insulated wires, which are mainly enameled wires, as magnet wires in the coils. Further, as described above, since a voltage almost twice as high as the inverter output voltage is applied to in inverter-related equipments, it becomes required in insulated wires to have minimized partial discharge deterioration due to the inverter surge.
In general, the partial discharge deterioration refers to a phenomenon in which an electric insulating material complexly receives: molecular chain scission deterioration, caused by collision with charged particles that have been generated by the partial discharge of the electric insulating material (discharge at a portion in which fine void defect and the like exist); sputtering deterioration; thermal fusion or thermal decomposition deterioration, caused by local temperature rise; chemical deterioration, caused by ozone generated due to discharge; or the like. Thus, thickness is lowered, in several cases, in the electric insulating materials which actually have been deteriorated by the partial discharge.
Like the case where the relative dielectric constant of almost resins which are ordinarily used as a material of the insulating layer exists between 3 and 4, there is no material whose relative dielectric constant is particularly low. Further, in practice, taking, into account, other characteristics to be required for the insulating layer (flexibility, heat resistance, solvent resistance, and the like), consistently a material, whose relative dielectric constant is low, cannot be always selected.
Recently, it is required to increase particularly a ratio (space factor) of the conductor cross-sectional area to the stator slot cross-sectional area. In view of this required characteristic, excellent flexibility is required for the insulated wire.
In addition to this, a working frequency of the electrical equipment coil is expanding to a GHz region. The higher the working frequency is, the larger the dielectric loss of an insulator portion of the insulated wire becomes. For lowering in dielectric characteristics or improvement in transmission characteristics, with a provision of a plurality of foamed insulating layers composed of thermoplastic resins, a means of making the foam density of the foamed insulating layer at a center conductor side to be higher is proposed (see Patent Literature 1), or a means of adjusting a foam density of the outer foamed insulating layer to the extent of 30 to 60% is proposed (see Patent Literature 2).